1. Field of the Invention
This invention relates to the art of thermoforming rigid plastic articles and more particularly plastic containers.
2. Description of Prior Art
It is well known in the art that rigid plastic containers are manufactured from a thermoplastic sheet. The sheet is stripped from a pre-manufactured roll, reheated to a suitable forming temperature then indexed horizontally through a vacuum and or pressure forming station. After fonning, the articles may be trimmed while still inside the form tool, or stripped as an integral part of the sheet then indexed through a shearing station where the articles are separated from the sheet. This type of process is commonly referred to as Flat Bed Thermoforming and is described in U.S. Pat. Nos. 3,470,281, 3,578,735 and 3,600,753.
U.S. Pat. Nos. 2,902,718, 3,181,202, 3,518,725, 3,667,889, 3,771,938 each describe a means to thermoform shallow containers from thermoplastic sheet. The sheet is stripped from a premanufactured roll and continuously fed over the surface of a rotating drum comprising a plurality of vacuum forming mold cavities. During the time that the sheet is on the drum it is reheated to a suitable forming temperature and a vacuum is systematically applied to each mold cavity to form the articles. These patents are limited to the production of shallow trays and lids. Furthermore, each machine's production speed is limited by each apparatus' lack of heat transfer efficiency which is due to the reliance on the circulation of ambient air across the formed sheet.
U.S. Pat. No. 4,061,706 describes a process for the continuous melt thermoforming of polymers. This process utilizes a continuous sheet of molten plastic that is extruded and vacuum formed on a continuous belt or a rotating drum comprising a plurality of mold cavities. By utilizing the residual heat from the extrusion process, the need to reheat the plastic sheet prior to thermoforming is eliminated. By its' own description this patent is limited to the production of shallow trays. Furthermore the use of porous metal for the construction of the mold cavities is not practical. Eventually gases from the molten sheet will condense in the pores and decrease the mold's ability to draw a vacuum and form a quality product.
U.S. Pat. Nos. 3,027,596 and 3,071,812 each describe an apparatus for vacuum forming dished and flat articles. Each patent utilizes a continuous sheet of molten plastic that is extruded and vacuum formed on a rotating drum comprising a plurality of mold cavities. By utilizing the residual heat from the extrusion process the need to reheat the plastic sheet prior to thermoforming eliminated. The problem with these patents is that they are limited to the production of shallow trays and lids. Since the mold cavitv's position is fixed, the removal of an article, like a drinking cup or food container as an integral part of the sheet, would result in a distorted flange, dented heel and scratched sidewall, especially with those articles with a depth greater than 50% of the opening of the container. Furthermore, each system's production speed is limited by the apparatus' lack of heat transfer efficiency. In known thermoforming methods, water is circulated around the form tool to provide proper heat transfer from the plastic article. U.S. Pat. No. 3,027,596 relies on the temperature of ambient air to cool the plastic. U.S. Pat. No. 3,071,812 relies on water circulating three surface laminations beneath the actual forming surface of the mold. Each laminated surface creates a level of insulation from the chilled water resulting in the loss of efficient heat transfer.
U.S. Pat. No. 4,235,579 describes a continuous method of manufacturing plastic articles. The molding station described in this patent utilizes a continuous annular tube of molten plastic which is drawn through a roll mechanism to collapse the tube into a flat, double walled web. Utilizing the residual heat from the extrusion process the web is pressure formed on a continuously rotating drum which holds a plurality of pressure boxes about its' circumference, each containing one assist plug assembly. The web is positioned over each pressure box assembly at a rate equal to the molten web's extrusion speed. The mold cavity which is an integral part of each pressure box is then pivoted over the web and clamped to the pressure box. The part is pressure formed and resides on the drum for approximately 180 degrees. At this point the mold pivots open to permit the web and formed article to be stripped from the drum. Because of the inertia's involved in operating a pivoting mold assembly, this system becomes cumbersome at higher speeds. This system does not utilize a vacuum to assist in forming the product, only pressure is used. This limits the amount of definition that can be imparted into the product and it also reduces the heat transfer efficiency of the mold cavity. Furthermore the system's production output is limited by the use of a narrow plastic web and a single lane of form tooling.
U.S. Pat. No. 4,421,712 describes a continuous rotary thermoforming system. This process utilizes a continuous sheet of molten plastic that is extruded between; a rotating drum comprising a plurality of vacuum forming mold cavities, and a caterpillar like array of tooling made up of a pressure box, assist plug assemblies and in some cases matched molds. By utilizing the residual heat from the extrusion process, the need to reheat the plastic sheet prior to thermoforming is eliminated. The complexity of this system dictates expensive re-tooling and maintenance costs. Furthermore this system is limited to the production of shallow trays. Since the mold cavity's position is fixed, the removal of an article like a drinking cup or food container as an integral part of the sheet (as shown in FIGS. 1, 3 and 17) would result in a distorted flange, dented heel and scratched sidewall, especially for those articles with a depth greater than 50% of the opening of the container.
U.S. Pat. No. 4,722,820 describes a process for the feeding of a molten thermoplastic web directly and continuously into a thermoforming machine. Molten thermoplastic material is required to be fed horizontally through a pair of temperature controlled rolls that gauge the material thickness and pre-cool the plastic sheet prior to entering the thermoformer. The material is fed horizontally from the rolls directly and continuously under uniform tension without any means to support the soft, sagging, sheet as it is fed into a thermoforming machine. Extruded polypropylene and other similar thermoplastics are susceptible to sagging as the material reaches and exceeds the crystalline melting point. Variations in the material's melt strength are caused by changes in extrusion temperature, roll temperatures, changes in extrusion rate, and variations in the material's reology. To compensate for these variables and maintain the uniform tension required to properly transfer the sheet to the thermoformer, the speed of the thermoformer must be maintained at a faster rate than the rolls to prevent the material from sagging and adversely affecting the gauge of the sheet. This is especially true when processing thick and heavy plastic sheets. Care must be taken to avoid excessive speed in the thermoformer so that the sheet is not over tensioned adversely affecting the performance of the rolls and causing a change the gauge of the sheet. Furthermore, when starting up the thermoforming line the machine operator's hands must take hold of the molten sheet as it exits the sheet extrusion die and thread it between the rotating rolls and then onto the rotating members of the thermoforming machine described in the patent as a series of moving punches and dies. This start up procedure is performed at the risk to the operators safety.
U.S. Pat. No. 4,994,229 describes a process for forming thermoplastic material. This invention provides a solution to the problems associated with U.S. Pat. No. 4,722,820 by providing a means of supporting the soft, sagging molten sheet and transporting it to a thermoformer via an intricate conveyor belt system. The mechanical complexity of this belt conveyor system is further complicated by the necessity to regulate the temperature and speed of the belt to slow down or speed up the rate of reheating the plastic sheet to ensure that the molten plastic does not wet the belt and adhere to it. Furthermore, reheating the material should substantially occur before it enters the forming station for uniform, stress-free forming to result. Additional energy is required to reheat the material's underside of the sheet, or the side of the sheet that was in contact with the belt. This prior art does not address the safety risks associated with an operator threading molten sheet from the extrusion die through a series of counter rotating rolls and into a thermoformer.
U.S. Pat. No. 5,106,567 describes a continuous rotary thermoforming system. This process utilizes a continuous sheet of molten polyethylene terephthalate plastic that is extruded and vacuum formed on a rotating drum comprising a plurality of mold cavities around its' circumference. The residual heat from the extrusion process eliminates the need to reheat the plastic sheet prior to thermoforming. This patent describes a unique method of distributing, vacuum forming, and then crystallizing polyethylene terephthalate. The problem with this patent is that it is limited to the production of shallow trays and lids. Since the mold cavity's position is fixed, the removal an article like a drinking cup or food container as an integral part of the sheet, would result in a distorted flange, dented heel and scratched sidewall. This is especially the case in those articles having a depth greater than 50% of the opening of the container. Therefore, the process described in this prior patent is limited to the production of shallow polyethylene terephthalate trays that have a degree of crystallinity of at least twenty percent.
The mechanical arrangements and processes of all the known prior art systems solely result in the manufacture a particular size and shape of plastic article at a specific extrusion throughput. A major deterrent to the proliferation in the use of these prior technologies is the inflexibility of the apparatus and the extreme expense of fabricating a customized forming tool, or an entirely new and unique forming apparatus to accommodate a particular extrusion throughput for a new and different size and shape of plastic article intended to be manufactured. The molds described in the prior art are an integral part of the forming apparatus and are not easily removed to reconfigure the forming apparatus for the production of an alternate size and shape of plastic article. Furthermore, a change in the size of a plastic article may require a radical change in the diameter of the forming wheel. A radical change in the forming wheel's diameter without changing the location of the forming wheel's axis can adversely affect the distance between the surface of the molds and the sheet extrusion die rendering the apparatus useless. If too great of a distance exists between the molds and extrusion die, adequate control over the width, thickness and positioning of the plastic sheet on the forming tool is lost. Certainly a small plastic article can be produced on a larger forming wheel but this would always be done at the expense of a negative finished product to scrap ratio and the subsequent creation of excess extruded sheet and forming scrap.
By the present invention an improved process and apparatus for thermoforming is provided whereby a wide variety of plastic articles such as drinking cups, closures and food containers, with depths greater than 50% of the opening of the container, can be manufactured in a continuous manner on a rotating form tool. Furthermore the present invention provides for a continuous thermoforming system that is not a permanently configured for a particular size and shape of plastic article, is easily and economically transformed via change tooling to produce different sizes and shapes of plastic articles with the added capability to utilize various sheet extrusion throughputs efficiently. The present invention also provides a unique configuration of forming apparatus that permits the safe and efficient start up and operation of an extrusion system, the threading of molten plastic sheet through a set or series of temperature control rolls and onto a continuous thermoformer.